98 explorer humming when warmed up????

[Reaper]

I have a 98 Explorer 4.0 SOHC 21000k and when the vehicle warms up I hear a humming sound and even feel it throughout the inside of the ****pit.... I raise the hood and I can barely hear it and feel it out there as well but it is transferred through the inside of the cab louder and harder. Any idea what might be happening cuz I have only had this explorer about 2 weeks and wife drives it all the time and just noticed it a couple of days ago....